1. Field of the Invention
The present invention relates to a metal halide discharge lamp, and more specifically, to a metal halide headlamp for vehicles, where adjustments, such as a current increasing, are achieved to obtain a predetermined brightness immediately after the lamp is turned on.
2. Background Art
When applying a metal halide lamp 90 shown in FIG. 4 of the type proposed in the present invention as a light source for a headlamp of a vehicle, the following approach has conventionally been adopted: A current that is a few times greater than a rated current for a stable state is passed across a pair of electrodes 91 to increase luminous intensity of the lamp which is insufficient immediately after the lamp is turned on; thereby the metal halide lamp 90 is rapidly heated to attain the stable state quickly.
When the electrode 91 is rapidly heated in the above lighting approach, a problem arises that the thermal expansion of silica glass constituting a sealing portion 92a of a bulb 92 cannot comply with the thermal expansion of the electrode 91 so that cracks occur in the sealing portion 92a, resulting in a leak failure.
To cope with this problem, a proposal has been made to prevent the occurrence of cracks in the sealing portion 92a by wrapping a coil as a buffer around an appropriate area of the electrode 91. In this approach, however, metal halides sealed in the discharge chamber 92b of the bulb 92 are accumulated on the coil as time goes, leading to the other problems of changes in the lamp characteristics and the degradation of durability.
Though Japanese Patent Application Laid-open No. Hei 6-223781 discloses an invention, in which beads are attached as a buffer around an electrode, this invention does not allow high precision in the positioning of the electrode and, as a result, cumbersome adjustments are required when installing headlamps. Problems have always remained to be solved in the prior art, regardless of any shapes of the lamp adopted in the prior art.